1. Field of the Invention
The present invention relates to a power-supply control circuit and a radio select call receiver. Specifically, the present invention relates to a control circuit for supplying power to a circuit with a small load and relates to improvement of a radio receiver provided with a sampling-operation function for reducing current consumption by the circuit.
2. Description of the Related Art
In recent years, transistor circuits driven by a low voltage have been employed as a variety of electronic devices, accompanying the increased density and the enhanced integration of semiconductor integrated circuits which are referred to hereafter as LSIs. In particular, such transistor circuits have been used to reduce the power consumption of battery-driven handy equipment. For example, the production of radio select call receivers is under way. A radio select call receiver executes message processing when it receives a radio call signal that matches its call code identification to be described later. Normally, a radio call signal is generated by a radio base station as carrier waves which are transmitted after being modulated by a call code signal.
The radio select call receiver employs a DC/DC converter for supplying a secondary power-supply voltage to a load circuit thereof. The DC/DC converter is controlled to operate in a stand-by mode. However, a capacitor connected to the output of the DC/DC converter lengthens the rise time of the output voltage. In addition, the switching operation of the DC/DC converter generates a ripple component on the secondary power-supply voltage.
Addressing the problems described above, a technology to which the present invention relates is explained. Let us take a radio select call receiver for carrying out message outputting operations based on a result of decoding a radio call signal S1 as an example. As shown in FIG. 1, the radio select call receiver comprises, among other components, a battery EB serving as a primary power supply, a radio reception unit 1, a wave-form shaping unit 2, a decoder 3, a central processing unit 4, a call output circuit 5, a call generator 6 and a DC/DC converter 7. It should be noted that the central processing unit 4 is abbreviated hereafter simply as the CPU.
The DC/DC converter 7 converts a primary power-supply voltage VBB output by the battery EB into a secondary power-supply voltage VSB which is then supplied to the radio reception unit 1, the wave-form shaping-circuit unit 2, the decoder 3, the CPU 4 and the call output circuit 5.
In addition, a switching element M2 is connected to the DC/DC converter 7. In a stand-by mode, the switching element M2 is turned on and off. It should be noted that the stand-by mode is abbreviated hereafter simply as the SB mode. Even though the location at which the switching element M2 is connected is not an issue; in the example shown in FIG. 1, the switching element M2 is connected between the DC/DC converter 7 and a ground line GND in order to simplify the explanation.
When the user turns on a main switch M1, the radio select call receiver functions, demodulating modulated carrier waves received from a radio station which serves as a transmitter of the carrier waves. The modulated carrier waves are electrical waves modulated by a radio call signal S1 in the radio station. To be more specific, the modulated carrier waves are received by the radio reception unit 1 employed by the radio select call receiver.
At that time, the carrier waves modulated by the radio call signal S1 are received in a sampling operation in the SB mode in order to reduce the power consumed by the radio select call receiver out off the battery EB. Powered by both the primary and secondary power-supply voltages VBB and VSB, the radio reception unit 1 carries out signal processing.
After undergoing a wave-form shaping operation in the wave-form shaping circuit unit 2, the radio call signal S1 is decoded by the decoder 3 to determine whether or not the radio call signal S1 matches a unique call code ID peculiarly assigned to this particular radio select call receiver. If a decoding result indicates that the radio call signal S1 matches the unique call code ID, a decoding-result signal is output by the CPU 4 to a call output circuit 5. The radio select call receiver then exits from the SB mode temporarily in order to prolong the period of time in which the DC/DC converter is generating an output. During the period extension, the call generator 6 is driven by the call output circuit 5 to display a message or to generate call sound.